A rear derailleur (rear speed shifter), for example, includes a chain guide supporting a guide pulley and a tension pulley for moving axially of a multiple free wheel by means of a control mechanism such as a pantograph link mechanism. A chain is engaged with the tension pulley and the guide pulley, and then engaged with the free wheel. When the chain guide is moved for shifting the guide pulley axially of the free wheel, the chain is shifted to a sprocket located right above the guide pulley. A rear derailleur of the above type is actuated by means of a control cable which is wound or paid out by operating a speed control lever.
The speed control lever generally comprises a bossed base portion having a cable winding groove, and an arm portion extending outward radially from the bossed base portion. The speed control lever is pivotally supported at its bossed base portion around a shaft provided at a suitable portion of a bicycle body. When the arm portion is pressed for rotating the bossed base portion, the control cable is wound around or paid out from the cable winding groove, generating an axial movement of the control cable corresponding to an amount of pivotal operation of the arm portion. The axial movement of the control cable causes a corresponding deformation of the control mechanism, causing the chain guide of the rear derailleur to be guided to a position axially of the free wheel corresponding to the amount of pivotal operation of the speed control lever.
The chain guide is moved continuously in response to an operation of the speed control lever. For this reason, a very high level of skill is required for quickly and assuredly guiding the chain guide to an appropriate position to each sprocket.
For example, when the chain guide is placed at an intermediate position between two adjacent sprockets, the chain is inclined as it runs from the guide pulley to the free wheel. As a result, the chain rubs against a guide plate, for instance, generating an unpleasant gear noise. A rider must therefore perform fine adjustment of the speed control lever so that the chain guide is guided at an appropriate position to a desired sprocket and that the gear noise or accompanying chain vibration ceases.
In order to improve operability of the speed change operation lever, there is provided a speed change operation assembly incorporating a click mechanism, wherein the control lever is held stepwise at rotational positions respectively corresponding to speed shift positions. In such a speed change operation assembly, the speed control lever can be held at predetermined rotational positions with assuredness. At each of the rotational positions, the chain guide or guide pulley is assuredly moved to an appropriate position to the corresponding sprocket, that is, the position right beneath that sprocket. This allows any rider to perform a speed change operation accurately and quickly.
For further improvement in speed change operability, an overshifting mechanism may sometimes be provided.
When an speed change operation is made, the overshifting mechanism causes the chain guide to be moved slightly more, shifting the chain beyond the predetermined position right beneath the desired sprocket, so that the chain is quickly engaged with a desired diametrically larger sprocket adjacent to the sprocket with which the chain was previously engaged. By providing the overshifting mechanism, speed change operability from a diametrically smaller sprocket to a diametrically larger sprocket is greatly improved.
For instance, the Japanese Utility Model Laid-open Hei 1-65783 discloses a click mechanism comprising an engaging member supported around a lever shaft, and a click plate provided with a plurality of engaging portions for successive engagement with the engaging member responsive to the rotational operation of a speed control lever. In this click mechanism, an overshifting mechanism includes predetermined play about an axis between the click plate and the lever, or a predetermined play with which the engaging member is held to the lever shaft.
However, such an overshifting mechanism provides an overshifting function at every speed position, causing inconveniences.
Especially, when the chain is shifted from a diametrically intermediate sprocket to a diametrically largest sprocket, the overshifting function often causes the chain to fall outward laterally of the diametrically largest sprocket. For this reason, in a conventional click mechanism, it was impossible to provide a sufficient amount of overshift when the chain is shifted from a diametrically smallest sprocket to a diametrically intermediate sprocket, resulting in insufficient overshifting function.
The click mechanism holds the speed control lever at predetermined rotational angular positions with positive feeling of assuredness as well as generating an engaging sound when the engaging portion is engaged with the engaging member. Thus, for performing a quick speed change operation, the rider must move the speed control lever further in the cable winding direction beyond its rotational position where the click mechanism generates the engaging sound to a position where the overshifting function is performed.
However, a rider is often mislead by the engaging sound to a belief that the speed change operation is completed, and therefore, stops operating the speed change lever when the generation of the engaging sound is heard. As a result, it was often likely that the overshifting function is not performed sufficiently, and in addition, speed change operability was even deteriorated.
Especially in a front speed shifter, since a rotational speed of the sprockets is slower than those on a rear wheel, it is necessary to hold the speed control lever at an overshift position for a significantly longer period of time. For this reason, it has been difficult to make sufficient advantage of the overshifting mechanism in a front speed shifter.